Metal Oxalates

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Contents

Introduction

Figure 1. Oxalate anion. Courtesy of Kemikungen, Wikimedia Commons

Metal oxalates are insoluble salts formed from the polyatomic anion, oxalate (fig. 1), and a metal cation. These alteration products, described as early as 1853 after being found on a column in the Parthenon (Liebig 1853), can be found on myriad substrates, including easel and wall paintings, polychrome stone and wood, stone monuments and architecture, and even metal, glass, rock art and paper. They have been the topic of considerable research for over three decades, particularly when encountered on outdoor sculpture, architecture, and wall paintings. While their presence on easel paintings has not been as thoroughly studied, there is growing interest in the issue as the number of identified cases increase (Bracco and Ciappi 2002, Sutherland et al 2005, 2006, Kahrim et al. 2009, Van der Snickt et al 2011, Breare 2017, Simek 2017, etc). Research generally focuses on calcium oxalate, which is the most frequently encountered oxalate salt across all media, although copper, cadmium, and zinc oxalate have also been identified. Researchers debate different theories explaining the chemistry underlying their formation, and it seems that more than one mechanism can result in these degradation products, depending on the local environment and materials present. Metal oxalates present a particularly taxing challenge to conservators as they are often visually disfiguring, yet are also notoriously difficult to remove given the compounds’ hardness and low solubility.

Characteristics

For easel paintings, these salts are usually found on or within degraded organic surface coatings above the paint layer(s), including natural resin varnishes and glazes, but have also been encountered within and between the paint layers themselves. Since calcium oxalate is the most prevalent metal oxalate associated with cultural materials, it is the most thoroughly described in conservation literature. The salt appears in two common mineral forms: whewellite (calcium oxalate monohydrate) and weddellite (calcium oxalate dihydrate). These salts are often present simultaneously on paintings.

In their pure form, these minerals are white or transparent. When encountered in paintings, they vary in appearance from white to off-white, light to dark brown or grey, and transparent to opaque due to their presence in heterogenous layers. They can form a thick crust that completely obscures the paint layers below, and have also been identified in areas with no apparent visual alteration. They are generally found on or within surface coatings, but can also be present within paint and ground layers. They can be evenly distributed or present in patches across a paint surface (fig. 2). Their broad variability in appearance renders them difficult to identify visually, and they require technical analysis to confirm their presence. They often resemble a degraded organic varnish, so their prevalence may be underestimated, given that they could be easily misidentified as such without confirmation from technical analysis. They often pose a significant challenge to both the visual and chemical interpretation of the paint layers below.

Figure 2. Oxalate-rich surface layers in the form of an uneven, opaque crust from a 15th century Italian tempera on wood painting, Virgin and Child Enthroned with Saints Christopher, Museum of Fine Arts, Boston, 37.410. Arrows indicate the thickest areas of the crust at the edge of the painting (left arrow) and areas containing azurite (right arrow). Oxalates were found in samples from regions with and without the crust. Courtesy Museum of Fine Arts, Boston.

Identification

Several different analytical techniques can be used to identify the presence of oxalates. Fourier transform infrared spectroscopy (FTIR) is one of the most commonly used techniques to identify calcium oxalate and its mineral form(s) (whewellite and/or weddellite). Raman spectroscopy, X-ray diffraction (XRD), and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) can accomplish the same identification, while also identifying other oxalate salts. Micro-Raman and ToF-SIMS are particularly useful as they provide high enough spatial resolution within a cross section to determine the distribution of salts throughout the ground, paint, and varnish layers. Oxalate can also be identified using gas chromatography mass spectrometry (GC-MS), scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX) and polarized light microscopy (PLM).

Mechanisms of Formation

The mechanism of metal oxalate formation on painted surfaces remains poorly understood despite significant research. These investigations mostly pertain to wall paintings and stone, and while environments and materials differ between these and easel paintings, the findings are still relevant. Regardless of the type of object, there must be sources for both oxalate and metal ions for these types of salts to form. Multiple explanations exist to account for the source of each component. Given that occurrences of oxalate salts vary so greatly in their appearance, concentration, and distribution, it is likely that different mechanisms are at play depending on local conditions (past and present) and materials involved.

Oxalate Source

Theories explaining the oxalate source fall into two categories: biological and chemical.

Biological

Bacteria, fungi, lichens, and algae all produce and secrete oxalic acid (of which oxalate is the conjugate base) as a metabolic product. There is strong evidence that oxalate-rich layers can result directly from microorganism activity on stone, metal, and wall paintings (Rosado et al. 2013). Biological underpinnings to oxalate formation are not surprising in context of outdoor sculpture, wall paintings, and buildings since these objects are commonly colonised by microflora. While easel paintings are generally more sheltered by being indoors, the potential for microbial growth still exists. Studies have shown that modern (Pavić et al. 2015) and traditional oil paintings (Seves et al. 1996) can support bacterial colonisation. Fungal growth is also possible on easel paintings (Sterflinger 2010, Koestler et al. 2003).

In addition to respiration, oxalate production in some species of fungi is believed to be a defence mechanism against toxic metals, including calcium and copper, where oxalate production rises with increasing concentrations of these metal ions in order to chelate and expel them (Gadd et al. 2014). Some fungal species produce calcium oxalate crystals independent of an external ion source (Guggiari et al. 2011). Further research is required to determine whether the fungal species capable of growing on a paint surface would possess these specific oxalate production mechanisms.

Chemical

A chemical mechanism could also explain this metal oxalate formation on painted surfaces, as oxalic acid is a degradation product of organic materials. These organic materials include many varnishes and other surface coatings, conservation products such as consolidants, and binders or additives in the paint and ground. Such degradation pathways have been induced in oil paint samples under experimental conditions (Colombini et al. 2002). This is the most likely mechanism at play in some stone examples where organic material is present (Cariati et al. 2000), and has been cited as the most plausible mechanism for easel paintings, given the ubiquity of organic materials in their layered structures and the lower likelihood of microbial growth compared to other materials.

In at least three cases where oxalate-rich layers formed on paintings, the degree of calcium oxalate formation was greater in areas of copper-containing pigments (Sutherland et al. 2005, Van der Snickt et al. 2011, Breare 2017). This may be due to the “defence mechanism” that produces increased levels of oxalate, or perhaps it is due to chemical differences in these regions, where selectively applied coatings or glazes (such as deteriorated organic lakes) were present in the copper-containing areas. Nevertheless, oxalates can be produced through chemical degradation or microbial activity.

Metal Ion Source

The source of metal ions varies from case to case. They may come from environmental deposits, material applied during past conservation efforts, or from the underlying paint and ground layers. In cases where the oxalates are present in a layer that remains consistent across a variety of pigments in the paint layers below, it would seem more likely that the calcium comes from environmental deposits or a coating with a calcium-containing material such as casein. In other cases, pigments likely play a role; they have been shown to interact with oxalic acid solutions to form metal oxalates under experimental conditions. For example, in one study, when various Ca, Cu, Pb, Fe, and Na containing pigments were tested, calcite and verdigris were most reactive, forming calcium and copper oxalates, respectively (Zoppi et al. 2010). Even when present in low concentrations, calcium ions have an extremely high propensity to form a salt with oxalate over a wide range of pH levels, which likely explains why calcium oxalate is by far the most common metal oxalate encountered.

In cases of oxalate salts other than calcium oxalate, pigments are the most likely source of metal ions. Copper oxalates have been found in the presence of azurite in mural paintings (Nevin et al. 2008, Lluveras et al. 2010), as well as on polychrome stone (Bordignon et al. 2008). Van Der Snickt et al. (2012) demonstrated how the interaction between oxalates in a varnish layer and cadmium ions in a cadmium yellow pigment in an underlying paint layer formed cadmium oxalate at the paint/varnish interface of a painting by Vincent van Gogh. Cadmium oxalates have also been found within paint layers of the oil-on-canvas painting Le bonheur de vivre by Henri Matisse, 1906-6 (Voras et al. 2015).

Treatment Methods

Metal oxalates present difficult cleaning challenges both chemically and ethically, as the salts are highly insoluble and tenacious and can involve the degradation of early or original materials such as varnishes and glaze layers. There is currently no cleaning method that addresses the removal or thinning of metal oxalate layers without considerable drawbacks. In the case of calcium oxalate, chelators targeting the Ca2+ ion such as citrate, EDTA, and DTPA have been used. These require considerable contact time and are typically applied in a gel or poultice. Generally, removal of the calcium oxalate is minimal before the contact time poses a risk to the paint layers (Bonino et al. 2015).

Another potential chemical strategy involves enzymes produced by fungi, bacteria, and plants. These enzymes, oxalate decarboxylase and oxalate oxidase, specifically target oxalate groups. This method remains theoretical until experimental testing can be carried out. Like chelators, the slow reaction rate may limit its effectiveness before the underlying paint becomes compromised.

Using lasers for cleaning paintings is an approach still in its infancy, although the Er:YAG laser has shown to be effective in reducing oxalate layers by the absorption of the 2940 nm wavelength by oxalate (Striova et al. 2015). Maintaining enough control of the process to homogeneously reduce the layer remains an issue, even when used by experienced operators and with the shortest available pulse times. Improvements in equipment may prove this technique viable in the future.

Currently, the most commonly used treatment is mechanical reduction, generally using a scalpel under the microscope. This is only appropriate when the oxalate salts are present above or within varnish layers, as when they have bound with the paint layer it is impossible to separate the two without damaging the paint (though thinning of the oxalate layer may be achieved). While this method may be effective and safe for the object in select cases, the technique is extremely time-consuming and may not be feasible for large-scale treatments.

Further Research

While research into the development of oxalate salts on other cultural heritage materials may help explain their formation on easel paintings, further research is needed to better understand the material and environmental factors specific to paintings. Likewise, given that salts can be so visually intrusive yet difficult to remove, conservation treatment methods warrant further investigation.

Bracco, P., and O. Ciappi. 2002. Technique of Execution, State of Conservation and Restoration of the Crucifix in Relation to Other Works by Giotto. The Painted Surface. In Ciatti, M., and Seidel, M. (ed.) 2002. Giotto: The Santa Maria Novella Crucifix, English ed. Florence: EDIFIR–Edizioni Firenze. 273–359.

Sutherland, K., B. Price, and A. Lins. 2006. The Characterization of Degraded, Oxalate-Rich Surface Layers on Paintings in The Seventh Biennial Gathering of the Infrared and Raman Users Group (IRUG7), Abstracts and Executive Summaries of Contributions. New York: Museum of Modern Art. 41-42.